This invention relates generally to tubing utilized in steam generating equipment and, more specifically, to a method of forming internally ribbed boiler tubes. The ribbing provides controlled internal flow disruption within the tubes to prevent stagnation of the steam bubbles that are formed during nucleate boiling; i.e., an operating condition wherein stagnating steam bubbles form an insulating layer which impedes the passage of the heat through the tube wall to the water flowing therein.
A major operating component of any conventional steam generating system is the boiler. The generation of steam is commonly accomplished by passage of water through a multiplicity of tubes, during which passage the water is sufficiently heated so as to cause it to change state; i.e., to change from a liquid to a vapor.
As the water flows through the tube, the water in closest proximity to the inner wall of the tube becomes heated by the heat being transmitted through the tube wall. This outer layer of water changes to steam. During this process of changing to steam, the first change which the outer layer of water undergoes is the formation therein of steam bubbles. The steam bubbles act as an insulating layer. Unless the steam bubbles are made to mix with the water in the tube, they will remain adjacent the tube wall, and take on the attributes of an insulating layer or film, thereby causing localized hot spots to develop along the tube wall. These hot spots, in turn, can cause overheating of the tube, and ultimately lead to tube failure. Additionally, unless they are made to mix, the steam bubbles by virtue of their insulating capability will also function to prevent further heating of the core of water, which is passing rapidly through the center the tube.
Thus, in order to achieve the rapid and efficient transfer of heat through the tube walls to the water flowing therein, a need exists to provide some form of means to break up the laminar flow of water through the tube and to effect the mixing of the outer layer of water and thereby also the steam bubbles entrained therein with the core of water flowing through the central region of the tube. One such means which has been employed in the prior art involves the usage of ribbing (lands or grooves) on the internal surfaces of the boiler tubes.
As regards the nature of the existing prior art relating to methods of making boiler tubes with ribbed inner wall surfaces, reference may be had to U.S. Pat. Nos. 3,088,494; 3,213,525; 3,272,961; 3,289,451 and 3,292,408. U.S. Pat. No. 3,088,494, which issued to P. H. Koch et al., is directed to providing a vapor generating tube that has its interior wall formed with helical lands and grooves, which are proportioned and arranged in a particular predetermined fashion. U.S. Pat. No. 3,213,525, which issued to W. M. Creighton et al., is directed to a method of forming an internal rib in the bore of a tube wherein material is removed from the inner tube wall by means of a cutting operation to form the subject ribbing. A still further example of these prior art teachings can be found in U.S. Pat. No. 3,272,961, which issued to L. A. Maier, Jr. et al., and wherein a method and apparatus are taught for making ribbed vapor generating tubes and in accordance with which a rib is deposited on the inside surface of the tube by means of a welding process. U.S. Pat. No. 3,289,451, which issued to P. H. Koch et al., is directed to a method and apparatus for forming internal helical ribbing in a tube wherein the internal ribbing is formed by means of a cold drawing operation. Finally, U.S. Pat. No. 3,292,408, which issued to J. R. Hill, is directed to a method of forming internally ribbed tubes wherein the tube is provided with an asymmetrical helical groove so as to facilitate removal of the forming tool from the tube.
Notwithstanding the existence of these prior art teachings, there is a need for a new and improved method of providing boiler tubes with a ribbed interior surface. The prior art methods that have been employed for this purpose have notable disadvantages and can be relatively expensive to employ.
One disadvantage in using these prior art methods and apparatus is the difficulty in successfully removing the forming member from the tube following completion of the metal deformation process. Generally, a member having a predetermined external configuration, such as a helical pattern, is inserted into the tube, and thereafter the tube is reduced in diameter such that the helical pattern on the member is formed in the inner wall of the tube. In order to remove this member from the tube it is necessary, because of the fact that the interior surface of the tube has been deformed so as to become essentially an exact complement of the member's external surface, to virtually unscrew the member from the tube to effect the removal of the former from the latter. The degree of difficulty in effecting the removal of the member from the tube depends on the length of the member which has been inserted into the tube, and the relative extent to which the pattern formed on the inner tube wall is a true complement of the pattern formed on the external surface of the aforesaid member.
Current methods of fabricating single lead rib (SLR) boiler tubes and multi-lead rib (MLR) boiler tubes often requires either mechanical or metallurgical deformation processing wherein a smooth tube is drawn over a slotted, rotating mandrel. During this process, the smooth interior surface of the tube is plastically deformed and forced to progressively conform to the slotted mandrel shape, thereby producing helical lead ribs along the tube length. This deformation process is not only difficult and costly but is also inherently limited in its ability to accurately produce rib cross-sectional shapes with the desired geometric detail and with the required dimensional accuracy. The conventional metallurgical processes are limited in their ability to produce optimized rib lead angles of 40° or more.
Further, the production of SLR and MLR tubes from high temperature, high strength, and deformation-resistant materials (such as alloy 800H), is very difficult using conventional deformation processing methods.